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Why do low wing aircraft have higher dihedral than high wing aircraft?
I have noticed and read in books that low wing aircraft have higher dihedral than high wing aircraft. Dihedral is design feature for lateral stability so should be needed for both designs. So why is it more pronounced in low wing designs?

There are several sources for a sideslip-induced rolling moment:

The dihedral angle $\nu$ of the wing, which will increase the local angle of attack $\alpha$ on the windward wing according to $\Delta\alpha = \beta\cdot sin\nu$; $\beta$ being the angle of sideslip,
The sweep angle $\varphi$ of the wing, which in a sideslip causes a de-sweeping of the airflow over the windward wing (and an increased sweep effect on the leeward wing). The local change in angle of attack is $\Delta\alpha = (cos(\varphi±\beta)-cos\varphi)\cdot(\alpha-\alpha_0)$ and is proportional to the angle of attack,
The crossflow around the fuselage (see sketch below for illustration, sorry, no simple formula here), and
The location of the vertical tail, or more precisely, the side force created on it by a sideslip angle in relation to the location of the center of gravity. This effect dictated the anhedral of aircraft like the F-104 Starfighter.

Please see this answer for a more complete explanation of effect 3. The sketch below is taken from the linked answer and shows a high and a low wing configuration in sideslip. The thin blue arrows indicate the sideways component of airflow $v_{\infty}\cdot sin\beta$.

In the end, some rolling due to sideslip is good, but too much must be avoided, and dihedral is used to complement the other effects such that the total is just right. A high wing already provides some positive rolling moment due to sideslip (negative $c_{l_{\beta}}$: When you deflect rudder to the left, the resulting sideslip should roll the aircraft to the left, too), so the wing doesn't need to contribute as much (by means of dihedral) as in low wing aircraft.
Dihedral (or for that matter, even a low center of gravity) will not roll the aircraft level: There is no aerodynamic way to achieve that! Dihedral will only give you a rolling moment when the aircraft sideslips.

Peter, Thanks for the reply. A clarification....both cases represent right bank, is that correct? So in a low wing craft, right bank (clockwise force) is opposed by rolling moment (counterclockwise force) due to dihedral which stabilizes the plane. Guessing from direction of rolling moment arrow in high wing craft, this would cause instability, hence lack of dihedral??? The blue arrows are confusing me. Is it meant to indicate direction of relative wind?

@yankeemike: Yes, the blue arrows indicate the sideways component of airflow. Both cases represent right sideslip, however. Dihedral or a high wing will not get you out of a bank, instead they will roll the aircraft away from the sideslip.

Yes but banking causes turning causes sideslip, therefore the roll torque generated by dihedral DOES tend to roll the aircraft towards wings-level.  A model glider with lots of dihedral will fly for hours with the average bank angle remaining near zero even with no pilot input.  Dihedral or high wing placement absolutely does tend to roll an aircraft out of a bank.

@quietflyer Right, but in inhabited aircraft too much dihedral quickly becomes uncomfortable.

High-wing aircraft already have better roll stability due to their vertical center of gravity located under the wing than low-wing aircraft (with vertical CG located above the wing).
From a source other than Wikipedia:

If the center of gravity is below the wing, the weight tends to restore the upright position. This is known as pendulum stability or the keel effect. If the CG is
  above the wing, the weight is destabilizing.

Wikipedia is not always right.

Also, more ground clearance.

"the weight tends to restore the upright position" - That's not true; gravity never exerts torque on an aircraft, or, for that matter, on anything else. (The exception is when tidal forces become significant, which never happens in Earth's atmosphere.) Aerodynamic forces can push an aircraft upright, but gravity cannot.

Peter Kampf is right--the airplane does not know the difference between gravity and the g forces (acceleration) that it experiences in a turn.  When the ball is centered, that means "gravity/acceleration" is pulling "straight down" on the airframe ("down" from the perspective of the airplane, not from the perspective of the horizon).  The airplane is not capable of "seeing" any other gravity, because gravity is the same as acceleration.  So when you are in a perfectly balanced turn, with the ball centered, the gravitational pull coming from earth is not relevant except as it is joined with the acceleration of the turn to create a new "down" direction for the airplane.
To understand this better, you need to visualize the same "mind experiment" that Einstein used when he came to understand gravity as merely acceleration and not anything more.  He put a man inside a closed box floating in space, with the man floating in the middle of the box. Then he hooked a rope to the box and accelerated the box smoothly in one direction, simulating gravity.  From the perspective of the man inside the closed box, it could have been sitting on the surface of the earth.  Everything about acceleration and gravity, from the perspective of the man in the box, was identical.  This helped Einstein to see that gravity was not "like" acceleration.  Gravity IS acceleration.  So when you use acceleration (changing directions in a turn; centrifugal force) to change the direction of "down" in a turn, the airplane absolutely does not know the difference between true "down" and its new "down" as adjusted by the turning acceleration.  Gravity can not "pull" the pendulum of the fuselage down in any other direction than the direction of "down" as indicated by the ball in your turn and bank indicator.  So the only way dihedral and the pendular effect of the fuselage can roll the wings back to level, is if the turn is not perfectly coordinated, i.e., the ball has rolled in the direction true "down" not the airplane's "down".  So maybe feet-flat-on-the-floor flying is not such a bad thing after all.


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Would a airfoil move backwards if you rotate it vertical
Lets say i have a wing with an engine attached to it like a propellor.
And now i point the nose of my airplane up untill it is vertical.
Would the wing still generate lift because of the air flowing past it. Resulting in a backwards movement of the entire airplane?
EDIT I am suggesting that the plane is hovering vertical not gaining or losing any altitude.
This may sound very vague, so here an image hopefully to make it clearer:

So just to clarify, the brown thing should resemble a propellor.
the yellow line the horizontal normal orientation of a wing.
and the red arrow the resulted direction of the airfoil.
and the green lines the airflow.
As you may have guessed i am not an expert when it comes to planes and the physics behind it all. So pls let me know if more information or clarification is needed.

I would say related, because maybe not totally clear in my question (will edit) I am talking when a plane is hovering vertical.

Yes, and if you don't counter you might accidentally pull a loop!

A practical example of your question is when a model aerobatic aircraft with a conventional aerofoil profile (ie, not symmetrical) is fitted with a sufficiently powerful motor that it can be made to hover in the vertical position. This is quite a common trick.
The answer is yes, there is a force in the direction of the red arrow.
The model might drift in that direction, although there are other aerodynamic forces, torque, centre of gravity & thrust vector issues that will affect the balance, and these will change as power is increased and the model goes from hover mode to vertical flight mode.

It would depend on the Angle Of Attack of the area of the wing in the propeller slipstream.  If the flow was at the wing's zero-lift AOA, there would be no lateral force. Aerobatic aircraft with symmetrical airfoils that are at zero incidence in the propeller slipstream would be that case.  Wayne Handley's Turbo Raven hovering act back in the 90s, where he would hover in front of the crowd for extended periods, would be like this (saw it a couple of times; my favourite airshow act of all time).
An airplane with a cambered airfoil, where the zero lift AOA was at a slight positive or negative angle to the stream would see a lateral component.  The pilot holding a hover like that would simply cancel that out instinctively with an small elevator input that slightly tilts the thrust vector to compensate.
This can be complicated by the need to use aileron as an anti-torque control during something like this (you need ailerons that are large and go right to the wing root so they are in the prop stream), which may modify the overall lateral lift effect.

Yes, that wing would create some sideways lift. Whether it would actually move depends on how you are holding your aeroplane vertical. Obviously it's not going to move if you've bolted it to a big, heavy, test stand. For an aerobatic plane prop-hanging, it would depend whether the aerofoil is symmetrical, and the angle at which it is fixed to the plane. The effect would be easy to counter by hovering not-quite vertically.
If you think about it, a propeller is just a vertical aerofoil that generates forward 'lift' when it's travelling up (and sideways and down).

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

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what is "Shock on shock heating"?
Can anybody enlighten me on the nature and physics behind what is referred to as Shock on Shock heating. Both , "Hypersonic: The Story of the North American X-15 , Jenkins and Landis" and "Lessons Learned from the X-43 Program published by AIAA", refer to this effect.
The static temperature jump though a shock is well described but this Shock on Shock heating seems to be an effect greater than the sum of it parts.
Both sources refer to flow within the Hypersonic regime.
thanks

Is this what you are looking for?

looks promising, I'll try and read it fully over the weekend, thanks

If you search for "Shock-on-shock heating" or "Shock-on-shock interactions" in quotes, you will find quite a few technical papers on the subject.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


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Are old illegal air transfer methods still feasible today?
A retired soldier told me he used to know people who smuggled jewelries, hard drugs, guns and even people from middle eastern countries to Europe, specifically, Germany! Via planes! In fact, he said some goods were too valuable for them to "risk it" by land transports.
He also added that the aircraft were operated by experienced World War II pilots who specialized in flying at high speed in very low altitudes to make deliveries for infantry units on distress, making them unnoticed on radars and unstoppable for air defense systems.
Are similar methods still possible to this date? if not, what has changed?
similar: How do people smuggle stuff with private airplanes?
NOTE: I have researched myself and so far couldn't find evidence that any sort of actual "improvements" were done in these 70+ years in this matter, it seems the security tightens only when it's demanded and new technologies are not consecutively being used to monitor the skies.
no duplicate of Can someone actually "fly under the radar"? (read comments)

Does this answer your question? Can someone actually "fly under the radar"?

@J.Hougaard I've read nearly all the references mentioned in that question. However i'm more or less looking for how they counter these illegal transfers in all these years of advancement in technology, not necessarily how some fly under the radars.

Does this answer your question? How do people smuggle stuff with private airplanes?

@RalphJ, i'm the writer of that question. I restructured it to prevent misinterpretations and no it doesn't answer my question.

you should explain in few simple words why this is not a dupe INSIDE the question. comments can be moved to chat and/or deleted easily.

Yes, it is possible to fly "under the radar".
The type of radio waves that are used by radar and similar equipment to track aircraft generally travel in straight lines. This means that, because of the curvature of the Earth, radars have blind spots at low altitudes when you are sufficiently far from the radar. See the top illustration (not to scale):

While radar coverage today is no doubt much better than it was during World War II, much airborne surveillance today (SSR, ADS, etc.) relies on cooperative targets - which means that tracking an aircraft is a two way communication; the radar sends out a request, and the aircraft replies. The pilot could easily turn off airborne surveillance equipment, thus becoming invisible to secondary radars, ADS and so on, but would still be visible on primary radars, which work by sending out a radio pulse which is reflected off the surface of an aircraft. My point is, primary radar coverage is limited, with more and more primary radars being phased out and replaced by secondary radars, so there are definitely plenty of areas where, if you fly low enough, you can avoid radar detection.

I only noticed the possible duplicate question after answering

Thanks for your answer, so basically if they don't implement the whole planet with some technology like TARS and rely on these primary equipments, people can still fly unnoticed in the skies.

Could you also explain your point on "relies on cooperative targets"? isn't not cooperating a ticket to being escorted by some fighter aircrafts in a few minutes!?

@INF By “cooperative targets” he’s referring to the target having a transponder that “cooperates” with the radar station by returning a signal of its own.

Could do to elaborate on "clutter", since that's a problem even for air-based radar such as AWACS.

@pjc50 most of these new technologies belong to the U.S., it seems either other countries are not so open about their military achievements or the U.S. invests generously in it's R&D departments.


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What are the correct fixes for the MAX's pitch control systems?
At this point, the ramifications of faulty data input from only one sensor into the MCAS system of the 737-MAX are well understood, having allegedly led to 346 deaths, losses of almost 10B USD for Boeing, losses of around another 1B USD for American and Southwest, skepticism about the FAA's oversight ability, and even a potential hit to overall U.S. GDP.
That's pretty bad. Even in today's business culture where many mistakes are whitewashed, explained away or "taken care" of by a few executive firings, it's still reasonable to conclude that had it known what was going to happen, the manufacturer Quality Control would have done anything in its power to identify and correct the issue before the planes first shipped.
Indeed, the concept (in hindsight) should have never left the drawing board.
Solutions could be as simple as releasing the software patch they claim is currently under test,  through programming not accepting data from only one sensor (not as an option), and better pilot training and education.
It could also be fixed by more organic solutions, such as :  adding several more AOA sensors, lengthening the fuselage, repositioning the engines (setting them a bit wider), and/or longer landing gear (see Constellation, Concorde, XB-70).
What would have to be done to the plane's design to make its aerodynamic handling equivalent or better than the 737 NG series (or any other modern airliner), without need of a late-stage corrective system?

IMO, MACS, if done right, is a viable solution, and is not less organic than any other options. Augmented control isn't anything new by this time. What's wrong is 1) MACS's lack of redundancy of key componens, especially THE most critical AOA sensor, 2) MACS's dangerous behavior when components fails, 3) general lack of usability and thoughtfulness, being a rushed duct tape solution  4) Boeing's withholding of information from pilots of MACS's behavior and failure modes.

I think, my point is, MAX's tragedy isn't a series of technical mishaps. It's a series of corporate crimes

Arguably, 1 and 2 (and partly 3) are "technical mishaps", and even the rest is not necessarily "corporate crime". Lack of proper judgement, yes, and I gues this is the crux of the question: what would be a better judgement. I agree with John's answer on details.

The Max's problems had nothing to do with stall. The larger more forward nacelles caused shifts in the aerodynamic neutral point, only discovered during testing, that had an effect similar to shifting the center of gravity aft, and if you were holding a certain amount of stick force, say in a turn, keeping that stick force in would cause to nose to drift up unexpectedly.  This happened in certain configurations and speeds.  It was as if a whole bunch of passengers suddenly moved aft in a group and you were holding 15 pounds of back pressure and suddenly that back pressure would make the nose rise when you didn't actually change anything you were doing, and you would have to ease off some of the back pressure to compensate.
The behaviour in itself did not prevent the airplane from being certified if pilots were trained to recognize it.  But it was enough of a deviation from the other '37s behaviour that it put the common type certification at risk.  This results in a very expensive supplemental training burden on Boeing's customers.  MCAS was a software bandaid that used the trim system to make the airplane's behaviour mimic other 737s in the particular regimes where this was happening.
Boeing used an even stranger software bandaid on the 787-8, where the FBW system actively runs the normally dormant outboard ailerons to fix a wing vibration problem.  In the age of FBW this isn't all that controversial.
Anyway, there was a very fatal defect in the architecture of the system where it took data from only one AOA vane, because whoever did the system safety analysis decided that the failure mode where the system malfuntions due to defective AOA input was a low criticality failure (I think it was only classed as Major, not Hazardous or Catastophic).  It was classed as being equivalent to a stab trim runaway and depended on the HSTAB QRH procedure to deal with.
This allowed a mathematical risk justification for taking a signal from only one AOA vane.  This helps with system dispatch reliability, because if you have dual inputs and must have both, that's "anti-redundant" from a dispatch reliability perspective (two inputs are half as reliable as one).  Changing the system to have only one input doubles your dispatch reliability and if it's considered not that big a deal safety wise, it makes sense to do so. That's the main motivation for doing that single input architecture even though it was only more or less wiring and software differences.
The alternative to MCAS would have been physical redesign, maybe moving the operating CG forward which would have required a larger tail, or some other physical change, maybe something to the nacelles that would spoil the lift they were creating, that would've driven costs up and made the airplane way less saleable, and it would probably have killed the program.
In any case, notwithstanding all the other cockroaches that have scurried out from under the rug as it was lifted in the investigation, if Boeing had taken signals from both AOAs and lived with the need to put up with dispatch delays caused by having dual inputs where both must be present, none of this whole fiasco would have happened and it would be merrily doing its thing.  The safety analysis that was used to justify a single AOA input (and the FAA's failure to catch it) is at the root of all of it, in my opinion (having participated in supporting roles in similar crises with another OEM over the years).  Too bad really.

As critical as these AOA sensors are, I would have 5, with a simple program to average the closest 3.  Even 2 is not enough.  With today's incredibly advanced flying computers, these 20th century engineers also may have reduced to size of the wrong fin.  Look at bird evolution.  They kept their (variable sweep and area) horizontal stabilizer, and, I recently noticed, may even use their webbed feet to help out!  Pitch control of the wing is a requirement that can't be designed away.

That would be wonderful, but by the time you were done over-redundacizing, the project would get scrapped as too insanely overcomplex and expensive.  The real world is a mathematical balancing act between risk/benefit/cost.  It seems cold and calculating, but it has to be or none of us would be able to afford a ticket.

Would 3 more sensors and a little BASIC be too expensive?

From what I've read, MCAS was designed to use two AOA sensors and a warning if they disagreed, but later that was changed into an optional upgrade. The very idea that a safety-critical system could be made optional (with no public documentation as to why it's needed) calls into question the entire process.

As I said, the defect was in the system safety analysis that established the risk category under which the mod would be certified which justified that sort of single point of failure.  Basically the analysis would label it 'sorta safety critical but not too critical".  I have little doubt there were strong opinions within Boeing that this was a bad idea, but somebody won out, to their regret now.  The secondary issue is the FAA being complacent in accepting Boeing's word.  This is another 'balancing act"; if the regulator demands independent proof of every assertion, nothing would get done.

@RobertDiGiovanni  If that sort of thinking was applied everywhere, which would be the case, yes.

An impression I had was that the ultimate "power" (for lack of better term) of the system changed during development, that is it's ability to stay activated and range of correction it can apply. The system runaway and latchup condition was enough to overpower sustained human input, regardless of what the safety implications of a disabled system (which justify minimal redundancy), it was not considered the safety implications of a runaway. I work with similar issues as well and the situation triggers deja Vu for me but luckily in my domain (factory equipment) it is rarely a safety concern.

@crasic yes the scope of the MCAS's authority evolved and grew and the original criticality classification was probably acceptable, but it should have been re-analyzed and upgraded from Major to Hazardous at some point.  I'm sure there was a robust internal debate over that between "factions" you might say, one representing pure safety concerns and one representing the customers' interests from a maintainablity perspective (I have personal experience with this in the industry).  Somebody didn't put their foot down when they should have even if it meant opposing senior management.


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How does propeller length, pitch, and airspeed affect efficiency?
tl;dr: trying to find propeller efficiency with thrust, velocity and power and see which propeller length to pitch ratio is efficient.
Hello, I'm a high school student working on an extended essay for my physics class. What I've basically done is took a bunch of propellers (7x4E, 7x5E, 7X6E, so on and so forth). Propellers of lengths: 7, 8, 9, 10 inches and pitches: 4, 5, 6, 7, 8. (Fixed pitch propellers).
I've taken these propellers and hooked them up to a motor and a weighing scale to try and find how much thrust they produce. Then, I put them on to a control line aircraft to see how fast the plane goes.
Basically, Im trying to find the efficiency of these propellers and the data I have is the thrust produced by the propeller at different power levels and the velocity at different power levels. I'd really like to know if there's a relationship that I could employ to find the efficiency!
It doesn't have to be complex! Just needs to work from a physics standpoint. My deadline's coming up soon and any help would be greatly appreciated.

This answer should give you the needed theoretical background.

I'm voting to close because of the general lack of quality of this question. I think it's great that HS students come here for information, but it's also incredibly important to meet side guidelines. This question will far outlive the student's HS career, and will linger around the internet for possibly decades. The question decreases the SNR at SE.Aviation, as the poor title and body clarity makes it difficult to understand without taking a lot of time to read it. This nukes search engine results. I would retract this vote if the OP could go through and clean up the text and title.

Paragraph 4 qualifies this question.  Let's give it some time to improve.

Suggestion: can you change the slant of your project, to trying to prove or falsify the idea that the propeller that is most efficient in the static situation, is also most efficient in flight?  Or maybe show that starting from knowing which one is efficient on the ground, you can predict that one with a higher pitch will be most efficient in flight?  Just brainstorming here, hope it helps.

Your best bet for demonstration purposes should be to take it to the extreme. Compare 6 foot propeller with a 30 foot helicopter rotor blade. They are both the same thing in the end. The 6 foot prop will make maybe 600 lbs of thrust with 150 hp, whereas a 30 ft "prop" turning way slower, will make 4x that.

@RobertDiGiovanni not saying the question, once you dig down into it, is without merit. But it's important for us to maintain good quality expression in the body and title, either by letting the OP know there's an important need, or clarifying it (as you did).

@KennSebesta appreciate that.  Props and power seems to be misunderstood by many, including me until some time was taken to break down the terms as Force × distance (torque) per second, and applying this to drag resistance at a velocity where drag force +  equals thrust force.  With props, the "× distance" cancels out with the prop radius.  With a car × distance cancels out with the gear ratio.  RPM relates to fuel flow.  Fuel flow relates to "power". Which brings us to prop efficiency.

You may already have enough data for a very good report.
But know that propeller efficiency changes with airspeed!
Expect the finer pitch (4,5) to produce more thrust on the test stand, but the coarser 6 or 7 will produce more thrust in the air because forward motion changes the angle of attack of the rotating propeller.
Propeller length depends on engine power.  A really small engine can not turn a huge propeller fast enough$^1$.  Reynolds number also plays a role. Efficiency decreases if the prop rotation speed is too slow.  Though it is better to "move a lot of air a little", there is a lower limit to efficient prop speed, with the upper limit being its critical Mach number.
So, just plot your results, include the horsepower (or kilowatt) of your engine, and get your good grade.
Top speed will be your best prop for cruising, but a finer pitch may be better for climbing (at a slower speed).
For a A grade, measure RPM of your prop on the test stand.  Tachometers are readily available for this purpose.  Feel free to share your raw data with us, so we may help interpret and draw conclusions.
$^1$ though the Wright Brothers may have disagreed.  They turned 2 huge propellers with a really small engine.  But, with fixed pitch this will produce maximum efficiency only within a very small speed range.  Variable pitch props help solve this, and extend the range of airspeeds at maximum efficiency$^2$.
$^2$ see V-22 Osprey.

Pitch, diameter, rake, cup, material and # of blades will need to be factored to match the manufacturer's optimal wide open rpm of the engine. Lugging or overrevving the engine is not anything you want. A test stand will not do this.
Too much pitch, cup, rake and diameter will lug the engine and too little of all or some, will overrev. Also a propeller thats gives good acceleration may over rev once it gets going... and on the other side, one that gives greater top speeds over a longer flight may never reach its optimal recommended rpm. It must be done on an aircraft in flight. Also prop slip, blade cavitation, aerodynamic drag, temperature and altitude will affect results.


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How do people smuggle stuff with private airplanes?
I get it, the question seems as if I'm reaching out for answers from experienced smugglers! But no. As a matter of fact, I'm trying to get answers on air surveillance & security.
Love it or hate it, these kind of smuggles are being done all over the world. However, recently I heard a story from a retired soldier stating that he used to know people who knew people who smuggled jewelries, hard drugs, guns and even people from middle eastern countries to Europe, specifically, Germany! Via planes!
In fact, he said some stuff was too valuable for them to "risk it" by land transports. He added that the pilots were experienced World War II pilots who specialized in flying at high speed in very low altitudes to make deliveries for infantry units on distress, making them unnoticed on radars and unstoppable for air defense systems.
So that's for World War II, which ended on 1945, so the question is, how can people still do this considering all the enhancements and improvements or even inventions (which some boldly have "foolproof" labels attached to their names) implemented on radar and satellite technologies? Especially on a country like Germany, with superb defense and military status?

I very much doubt if there are many "experienced WWII pilots" flying today, since even if they started in 1945 at the age of 15, that'd make them 90 today.

Related: https://aviation.stackexchange.com/q/74771/15311

You could have gone more quickly to the point. You question is basically "how do smuggler avoid all surveillance and aircraft control systems to illegally enter Europe?". The first half of the question only dilutes your point.

Unfortunately @jamesqf, you have put your energy on figuring the wrong aspect, as it was never stated in the question that those pilots are held accountable for these smuggles today... at least not from what i've heard

@INF: I'm sorry, but your question does say just that.  If that's not what you intended to say, perhaps you need to edit it.

Those who are very successful at this will be expert at it, and are not dumb enough to answer on an open and traceable internet site.

@jamesqf the last paragraph starts with "So that's for World War II", stating that the story that was just told IS from the PAST. and finally the question starts with "how can PEOPLE still do this", which the subject is unknown, since some people agree with you (judging by the upvotes) i would appreciate if you pointed out where in my question i'm pointing to any experienced pilots doing this today? your math here gave me a new [irrelevant yet interesting] outlook though not gonna lie ;)

BTW, this question was closed due to the question being opinion-based. I personally would love to know the counter measures different countries take based on resources and researches provided by other experienced members. As a great example @Dave here posted a very resourceful comment which by reaching the end of it i knew a lot about the policies different countries would take to interact with air smuggling. I request the post being reopened so more of these useful answers could be submitted.

@INF: You wrote "RECENTLY I heard a story" from a retired soldier who "knew people who knew people WHO SMUGGLED", not "who used to smuggle in the post-WWII era", strongly implying that it is something going on very recently, if not in the present.

You're right @jamesqf, i can see now how that part can be misleading, i appreciate the correction.

There are lots of ways to smuggle goods with planes and it depends what you are smuggling and the aircraft on which you are smuggling it. Smugglers often bank on the fact that most countries do not have or allocate the resources to search every aircraft throughly enough from top to bottom before every flight as that is simply impractical on some level. Some times all you need is a good audio equipment road case.
Generally you need to get past customs which happens on the ground and is its own set of issues. In the USA the Customs And Border Protection takes care of a lot of the searching. Smuggling sometimes happens on commercial aircraft and there is plenty of info on it here on the CBP website. In this case the goods are smuggled on aircraft that are very much on radar and flying at assigned ATC speeds. Sometimes they are even loaded by official personal. On occasion even the head honcho is involved, moving some strange things.
It is possible to "fly under the radar" but that also depends on where you are going and what their radar coverage is. Steps have been taken over the years to combat this issue. It seems that some of the smugglers have once again resorted to small aircraft.
Barry Seal, the inspiration for the movie American Made was historically a pretty prolific smuggler. The movie takes some poetic license but is based on some facts.

Seal used his knowledge of aviation to become the infamous smuggler he
  was. Once in U.S. airspace, Seal would drop to 500 feet and slowed to
  120 knots to mimic, on radar screens, helicopters which frequently
  flew from oil rigs to the coast.
Within U.S. airspace, Seal would have people on the ground monitor for
  any signs his planes were being tailed. If they were, the mission was
  aborted. If not, they would continue on to drop sites over the
  Louisiana bayou, where duffel bags full of cocaine were dropped into
  the swamp. Helicopters would pick up the contraband and take them to
  off-loading sites, and then on to Ochoa distributors in Miami by car
  or truck.

Smugglers are still attempting to move product in older, but fast, biz jets with fake or missing registrations. To avoid being discovered they are apparently destroying the planes after they use them sometimes.
There is a great article on some of the history of air smuggling here. Which dates back to the early days of aviation.
This is a fairly interesting story...

Just remember, aviation smuggling is Illegal very illegal

Thanks for replying, the links were helpful to get the general  "idea" of how smuggles are done and what is being done to stop them. About the TARS technology, first time i read about it i thought, that alone can help spot the majority of the untracked flights i was talking about (as it actually did in the U.S.). However considering that the TARS technology is exclusively for the U.S., i couldn't find any sources stating that Germany, as a subject in my second question, or any other country is also using the same technology. Which leaves them vulnerable to untracked air traffic.

People smuggle with private instead of commercial because when boarding a private plane, the security checks are less thorough and very breif. Sometimes, they don't even get checked!
But on commercial jets on whatnot, the security is held to very high standards. That should answer your question.

Sometimes?  At least in the US, make that never.  (Corporate aircraft might sometimes have their own checks, of course.)  The only check is at customs, if you arrive from a foreign country.  If you can avoid that, or drop your illegal cargo before landing, you're home free.


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Which 2-seat 4th generation fighter/engine/configuration (de-militarized) would the best supersonic long-range civilian private transport?
Seeing recently -with some surprise- that former military jets of fairly recent vintage can be bought e.g. here and operated by (skilled+licensed²+rich³) private individuals after all classified/restricted tech has been removed, I was wondering what plane would be best suited for a Max-Fuel configuration.
I think I use the right terminology and assumptions, but for clarification:

2-pilot-seat configuration: "Learner configuration", like in driving school, where both seats can "pilot".
I'm assuming, for every modern 2-seat airplane a "learner" version/configuration_kit exists.
Super-Cruise: I believe is a supersonic mode of travel without using the Afterburner
Gen-4: I believe this includes F16 and newer (and foreign equivalents)
Gen-3 and older: There aren't any Super-Cruise-capable specimens?
I believe, all civilian runways are long enough for Gen-4 planes
I'm assuming, using afterburner absolutely kills max range.

For example, Wikipedia says (ballpark numbers) an F-18 weighs 15 tons empty, and has a max take-off weight of 30 tons. Internal fuel capacity + maximum external fuel capacity is 12 tons. I'm assuming the remaining 3 tons are the ammo for the Vulcan Gun, flares, the pilot and other stuff you can drop or shoot in order to have a minimum onboard defense/offense power, and not just fuel. Wikipedia seems to say in the specs, that it can travel ~2300km with ~6tons of fuel.
I'm assuming, that a de-militarized version of this 15ton bird will be quite a lot lighter, i mean, no [gun, ammo, flares, armor, ECM, cameras, etc.] needed. I'm guesstimating that could slim the plane down to say 12 tons. I'm aware, that not all the remaining ~6 tons could be used as additional fuel weight, but by converting the gun/weapons bay to big tanks, and running fuel lines to the missile-mounts, surely a large chunk of that could be converted to fuel weight.
So, the above looks like an F18 learner could be "converted" to a max-range-model 2-seater "sports-car-for-the-weekend-trip" with a fuel capacity of ~16 tons and therefore a range of ~5700km. That gets you over every ocean.
Now all of the above does not take into account the fuel usage at Super-Cruise (iiuc), and I have no idea which plane/engine is the most "efficient" in this speed envelope.
SO, basically, my question is:
Which plane would a "bezos-rich" person buy, if he wanted something for super-sonic weekend trips with his (also former air-force-pilot) friend, and what modifications could/would be possible to extend (super-sonic) range to achieve the best average speed. Is it fast enough, or even possible (w.r.t. time-zones)  to have brunch on both sides of an ocean?
@ManuH pointed to a great example from pre-jet-times, the business exec version of the Cavalier_Mustang
@RonBeyer: Don Kirlin bought 40+ air-worthy F18s from the Australian Airforce
Edited to address comments.

Note that former fighters have already been transformed for business travel

It's just not possible in today's FAA world. The most you'll be able to get is an exhibition certificate for this aircraft, allowing you to fly to/from airshows and practice for airshows only. That is if you can buy one, which is almost assuredly no. There was an F-16 (in 2-halves) available, but not sure you could fly it for personal use, they cut it in half to "de militarize" it. There is a "fully operational" one, but it is only for sale to contracting firms.

I'd be pretty concerned with the source of that story, the RAF has never flown the F-16.

JetLease owns a few that they bought from Belgium and Netherlands, but they are only being marketed to defense contractors.

@ymb1 no, it doesn't. I know that people can buy them. I want to know which one to buy/modify/how to get the best average speed over a transoceanic range (somebody modified the original Topic line beyond recognition)

@ymb1 Soo, can you suggest a better SE-site?

@ymb1 I do not believe these to be duplicates (even in the light of my meta answer), because these two questions can not be correctly answered by the same answer.

Does this answer your question? Can civilians buy military aircraft?

"Best" is subjective. At the very least, you need to prioritize your criteria for the question to be fully answerable.
I'll give a short answer based on your current criteria:

Civilians can potentially buy any fighter jet, provided it's demilitarized, depending on their relationship with the government. The US certainly won't sell an F-22 or F-35 to just any individual or country, but George W. Bush might have a shot. Absolute monarchs, of which there are quite a few left, might buy a jet for their military and appropriate it to themselves.
No recent fighter jet is even remotely close to being flyable by someone with only a PPL, even with every endorsement out there. The ground is severely obstructed in these planes while landing, their stall speed is uncomfortably high, their control sensitivity out of the ballpark. The only civilian-piloted case for them would be a retired fighter pilot.
No fighter jet is capable of long-range supersonic flight. The supersonic range of all fighters is extremely short - a few hundred miles for the best-case. The only military aircraft capable of sustained supersonic flight are the B-1 and the Tu-160.
Not much weight can be removed from a fighter jet before it's easier to buy two fighter engines and build an experimental aircraft around them instead. The gun and the main radar is about it.
If one had to choose specifically a fighter jet, the best choice is perhaps the Su-34, thanks to its amenities, as well as its long range and large useful load.
Of course, with unlimited choice, the Concorde would still be the best. Out of military aircraft, the Tu-160 - it's got enough in common with a supersonic airliner to convert, and it's the largest.
If SST and bombers are out of the picture, you'll get where you're going fastest in a fast bizjet like the Global 8000. The F-18 is not suited for the job at all, as it's not even able to supercruise. A fighter jet can be fun to fly (if you can do that) for the performance, not for its usefulness as a transport.

Pretty sure that George H. W. Bush (#41) is beyond using any form of physical wings at all right now.  George W. Bush (#43) is probably who you had in mind, but I don't think he's shown interest in piloting aircraft lately, even though he earned his AF wings several decades ago.

@RalphJ Right, barking up the wrong Bush.

Point 3 is the most important one. +1.

Sorry, but this scenario isn't remotely going to happen.  Nothing with supercruise is going to be sold to even a Jeff Bezos type of individual.
Beyond that, even with a big enough motor to fly supersonic without AB, it's still not an efficient way to get from A to B.  I'd be shocked if even an F-22 could cross the Atlantic without tanker support, in any configuration, let alone overcoming the drag of being supersonic.  (And no, Jeff Bezos can't borrow a KC-135 from the Air Force for his jaunt.)
And somebody like Bezos wouldn't want to cross the ocean strapped in to an ejection seat anyway.  After having his fun in something like a T-33 or A-4 (those do fly, a few, in civilian hands), he'd land, hop out, and climb into (the back of) his G-650 or Global Express for the hop across the pond for dinner.  Much more pleasant way to spend the hours involved.

Wikipedia gives the F-22 a ferry range of 3220 km, and it is 5585 km from London to New York, so it will need help


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Can a runway have a displaced threshold and a closed section in the same area?
There is tricky question we weren't able to find a clear answer to.
Following roughness tests we decided to close part of the runway to all aircraft movements (1400 ft out of a total 3000 ft runway). The taxiway is located at the center of the runway (photo 1).
Actually the airport is certified under Canadian regulation (CARS 302) and this is what is indicated in this case :

Closed marking is placed at each end of the portion thereof, declared closed.
Runway threshold is temporarily displaced from the runway end, arrows are provided on the portion of the runway before the threshold.

It's not clear what the best practice is if we have the two situations at the same time; a displaced threshold, and a closed section in the same area.
For the moment, we decided to maintain the two type of marking at the same time; closed marking are installed on the side of the runway (photo 3).
This is a temporary measure. Phase 1 of corrective works was done. Roughness test still not acceptable, we have to come up with a phase 2 corrective works before this winter.
Has anyone seen this before and what do you do in this case? I'm also interested to see what do you do under FAA regulation.
Thanks

This photo show the marking this winter without the arrows. To be able to paint the arrows for displaced threshold marking we installed the closed marking on the side of the runway (see below).

Picture with displaced threshold and closed marking

Actually, yes it's a temporary action. We made a major cracks repairs but still have overruns on the Boeing Bump Index as those repairs weren't intended for runway profile correction. More repairs to come in one month otherwise these measures will become permanents until complete rehabilitation of the airport infrastructure next year.

Your case now looks like you have a closed grass runway next to a paved runway with usual displaced threshold suitable for takeoff from it or landing from an opposite direction. Very confusing. And, in any case, new threshold markings and runway numbers are highly desirable in a new position, without them it is even more confusing

In any case, threshold and runway numbers are highly desirable in a new position. Possibly it is the reason why pilots were confused with your initial variant.
At least by FAA regulations, if you don't want to allow takeoff from this area, it should not be a displaced threshold. I believe that you have a relocated threshold (part of runway closed for takeoff and landing). It can still be available for taxi though, but you don't have a taxiway from that end of a runway. It should look like this:

FAA reference is here, in "Partially closed runway" paragraph.
I saw partially closed runway in Europe as well, and it looked like that (actually they marked that part as a blast pad, and I saw that several times):

Unfortunately did not find anything similar for Canada (used Canadian AIM), but "relocated threshold" is mentioned in some accident investigation in Canada: report. And I still don't understand why the runway should not be marked just as "closed", without threshold displacement markings. Like here:

Arrows may confuse pilots, because it usually means that that part of a runway can be used for takeoff, or landing in opposite direction.

Transport Canada visited the airport for process inspection this week and they didn't say anything about the use of the two types of marking at the same time. we still have to monitor if some pilots use the closed area. Otherwise, for the moment it's ok the way it is.

This is what Lelystad airport in the Netherlands currently looks like. The runway is being extended but the extension is not yet open for operations, so it's closed with a displaced treshold, similar to what you're having there (albeit for different reasons) for the time being.

This situation is temporary, the complete runway and new taxiways are scheduled to open in 2020.
As a trainee pilot there, I can tell you it's sometimes confusing when approaching with such a very long stretch of asphalt before the treshold. I've twice misjudged my approach and had my instructor take over on short final as I was about to land on the closed portion which is longer than the active runway itself.

We added the arrows lately following many comments from pilots as only the threshold bar marking is not that simple to judge for approaching as you mention. But now, as other comments indicate, arrow does allow pilots to circulate in the closed area if they don't pat attention to the X installed on the side. Looking probably to change the arrow bu chevrons...


text:
What does this switch on Boeing Sky overhead bins do? Why is it always set before taxi?
The pivoting overhead luggage bins in the Boeing Sky interior have a tiny switch/lever on one side. In my experience, cabin crew always flips this switch on every single bin (as indicated by the arrow below) when closing them after boarding. At least KLM/Transavia does it as a standard procedure on 737NGs.
So what does this switch do?

Image adapted from Boeing Media Center

I'm not sure if this is more on topic here or on Travel SE (because it's more a passenger-facing thing). Opinions (in the form of close votes) welcome.

Well, you are asking about the functional purpose of a specific part of an airplane, so it ought to be fine.  It's not obvious that understanding this detail would benefit one's travel experience, so it's hard to argue that it would be a better fit for the other site.  Seems fine here.  We do spend a lot of time talking about obscure details at least in some instances!


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Why does it take so long to stop the rotor of a helicopter after landing?
Every time I see a helicopter (EC135 or the like) land on a hospital helipad next to my workplace, I wonder why there's maybe a minute? of waiting for the rotor to slowly spool down before offloading the patient can begin. I'm sure that a minute can make a real difference to someone requiring urgent transport to a hospital. I would thus assume that ideally, the crew would love to just engage the mother of all rotor brakes at the moment of touchdown and get the patient out five seconds later. (This is an exaggeration, of course five seconds are an unattainable ideal, meant just to illustrate the point.)
From an EC135 manual I understand that the rotor brake can only be used at low RPM, after the rotor has slowed down sufficiently on its own, which seems to be what takes the longest. Being an engineer myself, I don't think that those helicopters were designed by idiots. There clearly must be a good reason why helicopters able to stop the rotor in 10-20 seconds aren't common.

Given that it seems to me that the rotor is spun down with the blades near zero AoA, why not pitch them to a high-drag state instead to waste energy quickly? Perhaps set them to negative AoA to pump lots of air upwards, or feather them?
Would a rotor brake able to soak up all the energy quickly enough be unacceptably heavy? Or would the torque break the rotor shaft?
Is the medevac helicopter market (presumably the only one that would care about quick offloading) just too small to pay for designing and certifying a dedicated solution? (Are the potentially saved human lives not worth the paperwork?)

Momentum is high (and aviation is made by light as possible objects, so also not more strong then necessary), but also the hydraulic, cooling need an engine (and without APU, you need it running at low speed). -- But you do not need to stop rotors to enter/exit an helicopter: in fact if you are not sure on the capability of the terrain to keep the load of the heli, you keep the engine up.

Why wouldn't the pilot leave the rotor spinning and pull up on the collective a little to flex the blades upwards, but not enough to lift the aircraft off the ground, and offload the patient straight away?

Be glad one has all that rotor momentum if the engine fails and you need to land.

One minute is pretty much insignificant in patient transportation. Critical is the time to get the emergency people to the patient, to stabilise him so he would be able to survive the transport. Once the bleeding is stopped, the heart beating again, and the first drugs and intubation in the patient, you're in no rush. You can't do a cardiac massage while the ambulance is driving, or the heli flying. The medics are, in both cases, strapped to their seats, for good reason.

@Karl Surely there are also cases where the patient needs an urgent operation to survive; perhaps to stop some bad internal bleeding or something. I clearly know nothing about emergency medicine so I can't quantify how often it happens, but it just sounds unlikely that everyone can be stabilized already before reaching the hospital.

@TooTea It's very modern for mislead statisticans to do the math (often correctly) and come up with a study that says e.g. a one minute reduction in flight time will save so-and-so many hundreds of lifes worldwide. That's cargo-cult science, because the predicted improvement is significantly smaller than the uncertainty in predicting the future survival rate of medevac passengers.

@Karl I'm not sure I understand. Are you saying I'm making stuff up or trying to manipulate someone? I have openly stated that I don't have the data, do you? For all I know, such a really urgent situation could occur in 50% of the cases or in 1% of the cases, and the conclusion would be very different depending on that. If you know for sure it's the latter and thus it makes no sense at all to optimize for these situations, please post it as an answer.

@TooTea No, and I don't have data either. I'm saying that a reduction of one minute in typically one hour between accident and OR table will only matter at all for a smaller fraction of the cases, and even there it will only be statistically significant if you go for large numbers. There will be far better ways to spend money, to save a minute before patient transportation, where all patients profit, and much more. You can only spend a dollar once. ;)

Are the medical staff on the helicopter during spin down not doing anything with the casualty at all to get them ready for transfer out or are they reduced to retying their shoelaces during that idle time?

@civitas If I knew, I wouldn't have asked this question. If you know that the wait never matters, please post that as a proper answer.

"set them to negative AoA to pump lots of air upwards" - you risk sucking debris and people upwards.

@RobertDiGiovanni rotor momentum will not keep a helicopter flying for long...

@jpe61 my understanding is a helicopter can autorotate, with enough momentum to briefly hover before landing.

@TooTea Regarding urgent operation? It depends. For critical trauma cases, at least some services are just training the air ambulance doctors to perform the operations. There's at least a couple of cases of air ambulance doctors performing open heart surgery in a street to stabilize someone. Yes, its not ideal from an infection point of view, but that can be treated afterwards, if it happens.

yes @RobertDiGiovanni, a very brief moment.

@user1937198 Classical M.A.S.H stuff ;)

A minute isn't that long a time. Arranging the medevac flight plus the flight itself will typically take about 1 hour in one of the best healthcare systems. Urban medevac cycle times can be shorter, with 10-minute dispatch, but the most critical time is about reaching the patient.
Braking the rotor mechanically will convert its momentum into torque against the helicopter. That can cause the helicopter to skip on the ground, especially if it has just landed and still doesn't have its whole weight on the skids. Regular helicopter rotor brakes are mostly meant to prevent accidents from wind-induced spin. Additionally, conventional drivetrains are built for relatively moderate stress.
The only way to avoid the torque problem is to go coaxial or counter-rotating like the Chinook. Then the torque from the two rotors cancels out, and the energy can be absorbed by brakes.
If one was designing for medevac in a "cost no object" environment, they might go coaxial anyway, due to smaller rotor diameter and better control, which can accelerate missions by more than a minute due to easier or better-positioned landings.
Coaxial drivetrains are highly stressed to begin with, as they have to absorb the assymmetry in lateral forces from the two rotors, requiring oversized high-strength steel parts, driving up their cost and weight. Fast rotor brakes would add more weight, but at least the rest of the high-stress drivetrain is already a sunk cost.
Ultimately, you can put a price on the human life, and aviation and medical industries have it in the 5-10 million range - higher than most. If a minute makes a 0.1% difference, it's worth $10k per medevac under that high evaluation.
A custom coaxial helicopter with rotor brakes will be on the upper side of the price range, but an extra million would pay off over 100 missions. Another cost would be noise: coaxials cannot be made as quiet as conventional designs with fenestrons or NOTAR.

Depends how you define 'best' health systems. Urban and rural medical helicopter setups have very different targets. Norway is a rural setup, supporting urgent but not-super critical medivac where the roads aren't good enough and distances too far. Compare this to the London Urban air ambulance which has a target response time of roughly 10 minutes each direction within its operational area, but only takes highest critical cases where 15 minutes for a road ambulance to arrive would be too long/the patient needs to be in front of a doctor in under 15 minutes.

One thing with the time to reach the patient: Whilst patient unload has to wait for the rotors to stop, do they have to wait for the rotors to stop in the field before the crew in the helicopter disembark? Since that would mean that the rotor stopping time doesn't apply to that critical time to reach the patient. And needing the couple of minutes for patient unload is mitigated by stabilization by the crew prior to loading.

@user1937198 Trained crews are normally permitted to exit a helicopter with the rotors still spinning. Military medevac is done with the rotor spinning. But I'm not sure on civilian procedure - there's more equipment involved, and it's important that nothing is flapping that could be hit by the rotor.

"it's important that nothing is flapping that could be hit by the rotor"  Didn't think of that!  Very critical point

@Therac The medics don't walk out of the helicopter in a white coat, with a stethoscope around their neck. ;)

The survival rate of medevac patients probably has a global year-to year variation in the 10% range. Working on .1% improvements is a waste of time and money, and probably lifes of helicopter crews in this case, with new technology that's not really a clear improvement.

@Karl Are you sure about those numbers? There are hundreds or thousands of such flights a year in my small EU country. There's got to be at least hundreds of thousands of such flights worldwide. That should me more than enough for decent statistics.

@TooTea Yes, but a lot of air ambulance (whether helicopter or small plane) trips are not for trauma/rescue situations but for transport between hospitals or other situations where the extra speed is considered important but it isn't "every second counts". And arguably overused in many of the non-trauma situations due to high profits for the companies in that business (which is quite different from the emergency trauma situations, which are often handled by govt. or non-profits) - I remember reading a book about the high cost of healthcare that went into that topic quite extensively.

@manassehkatz-Moving2Codidact That's largely US-specific, due to the combination of insurance and litigation. If you use air ambulance when you didn't have to, you just bill the insurance. But if you don't and the patient dies in transit, even if it wouldn't have helped, you're on the hook for lawsuits. So - "they pay, we fly".

Your question makes a lot of sense from a theoretical perspective and I have no good answers for the technical questions like whether a supplemental brake is possible because that's not the sort of engineering I do.
I do have experience both as an engineer and as a first responder, and I have worked for a company that provides this service in the US.
These flights are extremely expensive as it is, because they meet the convergence of some already-strict standards and requirements.
For instance, you pretty much need a turbine helicopter because of the weight of (normally) 2 pilots, 2 care providers, 1 patient, and a lot of heavy medical equipment. Adding more cost to the helicopter makes the equation much harder. Ultimately, however priceless we (as a society) feel the cost of life is, we simply cannot endure infinite expense for everyone who needs it.
Another answer speaks to the response time of the helicopter. Keep in mind that emergency medicine in the field takes a shockingly long period of time. I have spent the better part of an hour clearing a landing site for a helicopter. Just making the decision to have the flight scheduled can take tens of minutes. If the patient was going to die in the time it took the rotors to spin down, that's unfortunate, but it's also how these things go.
Another axiom of first responders is this: never add to the body count. Everyone who is involved is a person with a family and this is their job (or they are a volunteer). The needs of the one person, who had an unfortunate problem like a car accident, cannot put everyone involved at unreasonable risk. Everyone involved is comfortable waiting for the rotors to spin down safely and naturally because they want to live. Many of them probably know someone firsthand who didn't, and didn't.

This is a fantastically interesting real-world actual-experience economic answer, thanks!  one note, as everyone has explained, there is a plain technical reason you can't brake a helicopter blade: it will make the blades (or perhaps the whole helicopter) destroy itself in to shards due to the torque, or at best, will "skip around" (all of which options would kill a few/many people) - - as has been explained in the other answers. (Of course, any unrealistic engineering challenge can be overcome with unrealistic money, but that is the "reason".)

@Fattie It sounds unrealistic that the blades would shatter if they were to be slowed down any faster than they currently are. The blades are able to carry several metric tons of load in flight and they certainly weigh at least an order of magnitude less than that, so they should survive a couple of g's of deceleration just fine. Their attachment to the rotor hub is likely a different story, since that's designed to handle the normal load acting in a very different direction so it might well be a bit too weak in the plane of the rotor, plus there's the leverage of a very long blade.

Time taken to make the decision is service and context specific. For a dedicated urban air ambulance service, 10 minutes to decide would be pointlessly paralyzing, with a conventional road ambulance already being most of the way there/already at the scene. 1 hour to helicopter landing would already have the patient in a hospital. But that's a very different service and set of goals when the entire purpose of the service is handing the patients who will die if not in front of a doctor in under 15 minutes vs handling patients who are multiple hours drive from a hospital.

@TooTea, for sure, that's possible, as I said "something bad would happen" - one expert below predicts the chopper overall may "skip" around (ie: killing everyone nearby!)

@user1937198 irrelevant. Most helicopter ambulance services don't work in urban centers where response times for regular ambulances are short AND there are few is any places for them to land. E.g. where I live the nearest place to land an helicopter would be a farm field about a 20 minute walk away, a field that usually has cows in it. By law an ambulance has to be at the scene after it's called within 15 minutes (though they often don't make that for lack of vehicles and crews).

 Helipcopters, when called, are usually for transport to specialist clinics (like burn units) further away where the helicopter can get there much faster than the ambulance despite the longer response times. So the vehicle gets there first, calls for the helicopter, and THEN the helicopter takes to the air

@jwenting Whilst most don't, those that do operate within urban areas exist, with the London Air Ambulance being one of the most extreme optimized cases, focused entirely critical trauma response within an urban area of less than 10 minutes flight time in radius. For that specific example, they have a setup that is supposed to take roughly 5 minutes to takeoff. Including a setup for listening in on ambulance dispatchers so they can make the call to dispatch the helicopter, before an ambulance arrives at the scene.

And 'an ambulance has to be at the scene in 15 minutes', and 'the patient is in front of a doctor qualified to perform surgery in 15 minutes' are two very different things. Which matters for things like if a patient is bleeding out from a stab wound.

If you were building a helicopter specifically for medivac and price was no object, there's nothing stopping you from doing it, but, the price will be a lot of weight, both for a brake large enough to stop the rotor from operating RPM, and the structural requirements for the rotor system itself to withstand the lead-direction bending loads on the blade roots plus the torque reaction at the top of the mast between the brake and rotor and brake and transmission, from applying heavy braking at high speed, that they don't normally have to withstand.
Once you add that sort of weight to an existing design, the merry-go-round starts because you lost some performance and need more power because of the ballast created by this very special feature, which needs more structure and more fuel, which needs more power, which needs...
So you have a helicopter that is quite a bit heavier, likely with a significant performance hit, saddled with a feature that nobody else in the industry wants, so it won't sell.

Okay, so basically an "all of the above" kind of answer. Regarding blade bending and torque limits, it feels to me that there should be plenty of headroom because spinning the rotor up before takeoff seems to be much quicker than stopping it after landing. Is there something making those two processes not symmetric?

I'm no expert on this, but from what little I've read/understand, I think a key issue is that the rotor is designed to basically spin freely in order to have even a tiny hope of being able to land safely in an engine-out situation. So the engine pushes the rotor in one direction and the rotor spins freely in that direction. But to slow it down you need either a large friction brake = weight + heat generated + wear & tear on rotor parts, etc., or some way to forcibly slow the rotor using the engine - but that goes against letting it run free.

@TooTea the spin up on start doesn't apply anything like the loads that would be applied (in the other direction) if you wanted to slow the rotor down quickly from the green arc.

@too re action being reversable, try spinning up a length of rope with your arm and then stop. As you accelerate the blades the centrifugal forces are pulling them straighter, but bringing them to a stop risks them getting all twisted as they unload. That said, pretty sure some ship based helicopters do have a blade brake to make deck operations easier, but those birds need robust blades to handle folding anyway.

"some way to forcibly slow the rotor using the engine - but that goes against letting it run free." - a gearbox? Depending on how the one-way linkage works, integrating a mechanism to switch it to both ways might be the easiest option.

@TooTea At least in my experience with helicopters, it takes a relatively long time for them to spin up, too, though I'm not sure how much of this is due to nearly all of the ones I've seen being turbine-powered vs. actual design requirements of the rotors. Starting up a turbofan engine on a fixed-wing aircraft also usually takes a while.

@JohnDvorak a gearbox is a brake.  torque is torque.  You can't beat physics in a non-Star Trek universe.

@reirab Fair point, but an EC135 can apparently take off about 70 seconds after engine start and I'm not sure the rotor was steadily accelerating all that time, so it feels like getting the reverse done in at most a minute should be technically feasible.

@JohnDvorak Helicopters typically use a https://en.wikipedia.org/wiki/Sprag_clutch to connect the transmission to the rotors, so that if the engine or transmission seizes, the rotor will still be able to autorotate. And thats not a mechanism that can really be made two ways without undermining the simplicity and reliability which is a key feature.

Addressing this subquestion that the other answers haven't included yet:

Given that it seems to me that the rotor is spun down with the blades near zero AoA, why not pitch them to a high-drag state instead to waste energy quickly? Perhaps set them to negative AoA to pump lots of air upwards, or feather them?

Unlike a mechanical brake on the axis, this air braking wouldn't generate extra torque on the helicopter. From physics perspective it is very feasible and could slow down the rotor faster.
However the mechanical design of most helicopters limits the collective movement to a range of about 0° to 20-40° (some models less, some models more). Thus there is no setting that would increase drag without creating lift. Extra lift would increase the risk of sideways movement or spinning due to motor braking torque and wind conditions.
Some helicopters like Westland Lynx do have negative collective, which is mostly used for landings on ship decks. Typically in such a scenario you would keep the rotor spinning until the helicopter can be secured by other means. Supporting negative collective requires multiple design changes:

More movement range in control rods and swashplate
Larger rotor clearance on the body, as the blades are deflected downwards
Rotor head bearings that can handle high axial forces in both directions

If the helicopter has the feature, I don't think there is anything that would stop you from using negative collective for ground landings. It's not a standard procedure and could result in increased wear.
On the Lynx the maximum negative pitch is about 2 degrees. Near 0 degrees airfoil drag increases slowly. Typically at +-5° the drag coefficient is about double of what it is at 0°. On the Lynx, using maximum negative pitch would probably increase the drag by at most 25%, so you wouldn't really save much time at all.
Turning the blades sideways (90° pitch) would greatly increase the drag and result in a rapid stop. But moving from the nearly 0° pitch at the moment of landing to this perpendicular configuration would temporarily cause large forces, again increasing the risk of movement on the landing pad.

Thanks, this also answers my unstated sub-question of "why can't helicopters "flare" the rotor like a fixed-wing landing" (gradually decreasing rotor RPM while in air and increasing collective so that there's as little excess momentum in the rotor at touchdown as possible) instead of using a giant constant-speed prop". Yeah, I obviously don't know anything about rotorcraft.

@TooTea I think technically they could, as that is kind of what you do in an autorotation landing if the propeller RPM starts to drop. But then you have much less control over altitude, because changing the motor speed takes time.

There is no need.
For pickup, helicopters can and do land within 1 meter of the patient (sorry for German).
Once inside the helicopter, the patient is already under medical care, so they should be out of the every-second-counts status in most cases.
I do see videos where medical personnel walks under a moving rotor, though this is on pickup, not on transfer into the hospital.

+1: rotor tilt can be neutral, i.e. rotor can turn with very limited air movement. The 1-minute delay between touchdown and people going out of the helicopter may be related not to rotor spinning, but to remove whatever leashes were installed on injured people for the flight phase.

You have a point, so yes, the medics do walk under the rotor. I'm less sure if it's safe for a patient, who might have tubes and devices attached to them, which could be pulled or pushed around by rotor wash.


text:
What is the rejected takeoff procedure for a blown tire?
When I got a tire/wheel issue during takeoff, how should I execute the rejected takeoff procedure? Is it different than the typical rejected takeoff procedure for an engine out that is applying the max brake?

Are you asking for something like a PA44 or an A380?

Could I know what are the differences?

With smaller planes, if you blow a tire on takeoff and have enough runway left to abort, then you should do so. Staying on the ground can be a much better option than a go-around and landing with a flat. Chop that throttle and “fly” the aircraft until it comes to full stop. That said, there will be certain issues to consider based on which wheel is the problem.
On a tricycle gear plane, popping a main will generally cause the plane to pull in that direction, and standing on the brakes aggressively can make it worse. It may require more gentle braking to maintain directional control and avoid going off the side of the runway, and this will, of course, increase stopping distance. If it’s the nose wheel that goes, directional control may be erratic and it might help to hold aft yoke to take some weight off the front gear as you apply brakes. If the plane has differential braking, you might be able to steer the plane with more authority by using stiffer pressure on one brake-side or the other.
If you’re in a taildragger, you may be straight out of luck. Keep those brake inputs punchy, and hopefully you can avoid a ground loop.
Most modern airliners have multiple tires on each landing gear strut, and the tires are designed to handle the load if one of the tires is compromised.
If the pilots know a tire has failed during takeoff at low speed, they will abort the takeoff using a prescribed procedure. At high speed, they will typically take off, perform a go-around, and return to land for a safety inspection.

If the crew of an airliner knows that a tire has blown during the takeoff roll too late to reject the takeoff, they will not retract the landing gear since a blown tire can distort and cause the gear to be jammed in the wheel well.

I was wondering, I guess all modern airliners have some sort of detector in each and every tire that knows about blowouts?  (And perhaps tire pressure generally??)  Or no?

@Fattie: Many modern airliners may have some version of a tire pressure monitoring system (TPMS) or tire pressure indicator system (TPIS), but not all of them. Plenty still rely on feel, abnormal brake temperature readings or outside observation to diagnose a problem.

@AaronHolmes - thanks, fascinating


text:
Are the GPWS callouts recorded with real people or computer generated?
I absolutely love the tone of the GPWS callouts in the Boeing 737NG, and I was wondering whether the callouts where computer generated or a recording of a brilliant voice actor.

Since the callouts are "fixed" and don't change it's easier and cheaper to design, manufacture, and certify the system using recorded voices. For each one, the software just needs to play the appropriate audio file.
I used to work on TCAS and the callouts were all just .wav files. Sometimes companies will hire voice actors (or subcontract to a recording company). We had an employee record the files.
Computer generated voice is useful where the content is highly variable such as the D-ATIS. (I know, it's a ground system but the concept applies.)

They should totally use celebrities for the voices!

@TomMcW  Some of the voice actors become celebrities in their own right.  The voice of "Bitchin' Betty" on the F/A-18, Leslie Shook, retired just a few years ago. Here's a link to the Boeing announcement.

Any idea who is the 737NG 's voice actor?

@lpydawa  No, sorry.  The 737 has a federated system.  Most Voice alerts come from the TAWS, which is a Honeywell product. TCAS and Weather radar (predictive windshear) each provide their own alerts. They could be Honeywell or Rockwell Collins or less likely either ACSS or Thales.


text:
On the 737NG, after deployment of spoilers automatically after landing, do the spoilers automatically go back down?
So, I was just wondering whether the spoilers on a 737-800, after automatic deployment when landing, will go back down automatically, or need manual input by pilots for the spoilers to go down.

The spoilers on the 737 after auto deploy will re-stow automatically based on TLP (thrust lever position). This is a function for a rejected landing which will automatically lower the boards & retract the levers for go around... most pilots after taxi off the runway quickly advance the throttle to let the actuator reset the system in leu of pushing the lever forward (it has a clutch in the linkage to allow manual positioning as well)

To add to Michael's answer, on pretty much all modern airliners they retract automatically after completion of the landing and are disarmed until re-armed on a subsequent take-off. There will also be some kind of manual override to manually disarm them.
There is usually a timer cycle of say 30 or 40 seconds after deployment, while still on the ground, at which point they retract and disarm.  It's the end of this timer cycle you are seeing when the spoilers come down as you are pulling onto the taxiway.

It should depend on the aircraft.  In the EA-6B Prowler the spoilers deployed when 3 conditions were met:  Switch armed, weight on wheels, and throttles at the idle stop.  (this presumes hydraulic pressure is available to actuate them...)
Adding any amount of power, even just enough to taxi, would cause them to retract if they were armed.  The post landing checklist called for the switch to be turned off, so they would then retract and not fluctuate up and down during taxi due to throttle movement.
I would imagine others work similar to this, but if you want a more detailed answer for a certain aircraft you will have to specify which particular one you are interested in.

I'm interested in the 737-800


text:
How do the vortices affect the flow of the horizontal stabilizer upon leaving the wing?
Other than the downwash effect, what other factors that affect the flow to the horizontal stabilizer? such as Dynamic Pressure? Air Flow Disturbance?

The answer depends on the relative position of both. If the wake from the wing directly hits the elevator, its reduced dynamic pressure will reduce elevator effectiveness.
Next, when near the stall angle of attack, the wing root will already exhibit some flow separation, and the turbulence of that flow will show up in buffeting on the tail. This is sometimes desired as a stall warning, but having the tail move into the wing wake just when the aircraft stalls is generally a bad idea.
To address the lift-related vortices: Those will reduce the angle of attack change on the tail relative to those on the wing. The increased strength of the downwash speed with lower flight speed (and, consequently, higher angle of attack) means that the effective angle of attack on the tail will be reduced. This narrows the range of angles of attack in which the tail needs to work and helps to ensure that the tail will show a linear response to changes in the flow even when the wing leaves the linear range already (in other words: When the wing stalls.)


text:
How do the vortices of a fixed wing and horizontal stabilizer interact?
What interaction of vortices between the fixed wing and horizontal stabilizer?
How do the vortices affect the flow of the horizontal stabilizer upon leaving the wing?
when will the downwash effort affect the flow, apart from not having clean-flow?
Does the downwash decrease dynamic pressure the reduce the lift?
Does the Airflow be affected after leaving the wing?

Hi Andy, since you're new, you might not be aware, but including several questions in the same post is generally not well received, because it is easy to make the question quite broad (and thus not good for the site). Please try to focus on one specific aspect per question.

Understand what you mean, I will ask the question individually. Thanks

The vortices coming off the ends of the wing are outside the width of the stabilator or stabilizer/elevator, thus there would be little interaction.
In my plane for example, the wings are 35 feet 6 inches wide, the stabilator only 11 feet 10 inches.
Thus the end of the wing is 17.25 feet from centerline, the end of the stabilator at 5.9 feet, thus 11+ feet of clearance.

I disagree - I don't see the wingtip vortices interacting with the stabilizer. Maybe the wingtip and tail vortices interact with each other behind the plane somewhere, but that was not the question.

Not necessarily specific to your model, but the more powerful vortex generators on some wings definitely DO produce vortices that interact w/ the horizontal stab.

I do Agree with Jihyun.
As the question was, How do the vortices affect the flow of the horizontal stabilizer upon leaving the wing?
Does the downwash decrease dynamic pressure the reduce the lift?
Does the Airflow be affected after leaving the wing?


text:
What kind of flight termination measures are taken for supersonic aircraft?
Having recently read  an article that hyper-sonic airplanes carry enough energy that just flying into some target makes them as powerful as some missiles and I wonder if the supersonic planes have been around for so long, what measures were taken if they misbehaved or went out of control. I've heard of crashes with pilots, but what about terminating the aircraft mid-air after getting the pilot to safely eject. With more UAVs and drones being used, what measures are taken to terminate the flight of such a system in case of a loss of control? I know of such measures for rockets but am unaware for flights.(Commercial ones have FAA guidelines or other national ones to follow)
P.S. Are there any books on such design or discussions about such situations. Please recommend.

I think "safely eject" from a supersonic (much less hypersonic) aircraft is a bit of a stretch. Only one person is known to have survived that: https://fighterjetsworld.com/2018/06/13/f-15-pilot-ejected-while-flying-at-supersonic-speeds/

Usually once a super-sonic aircraft departs controlled flight, the "destruction" is assured.

I fail to see why (relatively small) hyper-sonic airplanes would pose a larger threat than a big commercial airliner out of control. The total energy an A380 carries combined with full fuel tanks right after takeoff is certainly higher compared to a UAV or drone.

Aircraft like the B-58 and XB-70 Valkyrie were equipped with escape pods to aid in super sonic ejection. The pod its self came down around the seat when activated the only movement that appears to have been required was bringing you legs up a bit. Heres some video of the capsule in testing.

(source)
You can find some more info on the only XB-70 capsule ejection in the answer here. There is also some interesting info on the topic in this Physics.SE answer.
Ultimately getting lower and slower may be your only option as was the case during an SR-71 ejection.

Since the chances to survive an ejection at Mach 3.18 and 78,000 feet
  weren’t very good, Weaver and Zwayer decided to stay with the aircraft
  to restore control until they reached a lower speed and altitude, but
  the cumulative effects of system malfunctions exceeded flight control
  authority. Everything seemed to unfold in slow motion, even if the
  time from event onset to catastrophic departure from controlled flight
  was only two to three seconds.
It was at that point that the pressure suit proved to be very effective protection for Weaver. In fact, once it was inflated, an emergency oxygen cylinder in the seat kit attached to the parachute harness was functioning. It not only supplied breathing oxygen, but also pressurized the suit, preventing Weaver’s blood from boiling at the extremely high altitude. In this way the suit’s pressurization had also provided physical protection from intense buffeting and g-forces. That inflated suit had become like a tiny escape capsule.

There is some interesting footage of wind blast testing on flight gear and seats for the F-106 aircraft you can find here. According to some of this data the helmet and flight suits were capable of surviving wind forces in and around Mach 1.3 and the seat was capable of ejecting at those speeds. However in the case of the F-106 line of aircraft it was ultimately deemed that 0-0 ejections were more likely (and a higher priority) than high altitude high speed ejections. Since no seats were capable of both ejections the 0-0 seats were ultimately favored. (check out this for more info)


text:
Poor airfoil choice for wind turbine
I am getting involved in an outreach project where I can modify a small wind turbine with different wing geometries. Are there examples of standard airfoil designs with particularly poor Cl/Cd performance? This way I am hoping to demonstrate the impact of airfoil design (by comparison with better blade profiles).
Edit: Just to be clear, I am looking for something that looks like an airfoil - flat plates and round trailing edges obviously work but "look" wrong, hence the need for a "standard" airfoil design.
Many thanks!

Which level of fidelity has your model?

@sophit Thank you for your reply. If by fidelity you mean CFD then DNS :). This project will be experimental - I will be 3-D printing the blades. Fairly low Reynolds number though

Use a ordinary airfoil but backwards?

Uh, then you also have to consider stall characteristics, unsteady characteristics and surface finishing (dirt) of the leading edge... I suppose you'd better read some appropriate book like Leishman, which also contain a section about wind turbine aerodynamics

So basically you are looking for a legitimate airfoil that will be operating in wrong regime (reynolds) when put into a windmill?

Related: the opposite of https://aviation.stackexchange.com/questions/50689/how-to-pick-a-good-airfoil-based-on-cl-and-cd

Why not just take all the camber out, or use reverse camber to create negative CL?

@TomWaits: I'd suggest to use a standard NACA 0012. It was the first airfoil used on helicopters (and possibly wind turbines) and it has been superseded by more and more efficient airfoils. So you have also a technical and historical background for your choice.

In current form this is a terrible question without an answer. But interpreting a bit, I think what you really need is just some base level standardized airfoil profile to use for comparing other designs. In which case most people would start with a NACA 4 digit. Then being a turbine the airspeed changes from hub to tip so you must decide if you want twist or taper in the blade, and you could try installing it backwards or at odd angles of attack to see the negative effects of flawed engineering. Thickness matters aerodyn. and structurally, blades bending into the support pole is a problem.

Deisigning a wind turbine was actually my senior capstone for aerospace engineering at UMCP (I was on a team that competed in the USDOE Collegiate Wind Competition).  We used an open source program called QBlade to help inform our blade design (I actually took point on this).  What we found was quite interesting.  We tested a couple standard wing airfoils including the Clark-Y and NACA 0012.  These airfoils produced very poor power curves for a conventional horizontal axis wind turbine (HAWT).
Separately, there are airfoils out there designed specifically for HAWT applications.  NREL has designed the S807, which is reasonable, but for small scale applications actually doesn't work very well.  Then there's the SG6043, which my team ultimately settled on.  This airfoil is very good in small scale applications.  In our design work, the SG6043 produced a pretty stable power curve across a wide range of TSRs and windspeeds.
Overall, you'll notice that one key difference between the conventional wing airfoils and the wind turbine airfoils is that the L/D curves in the wind turbine foils is more symmetric about alpha == 0.
The 2021 UMCP design report is available from the USDOE, and goes into more detail about how we came to these conclusions.

Bravo for a good choice.  By chance, did you get around to trying a jib sail?


text:
What are the forces acting on an air particle along curved streamline(wing upper surface)?
What forces act at air particle at curved streamline (at first 1/4 upper wing surface) , looking from inertial and non-inertial frame of reference? (show free body diagram)

Pressure gradient normal to the local flow direction balance the centrifugal force associated with the curvature of flow..
Which Centrifugal force,inertial or reactive?

Is there a specific problem you are trying to solve?

no i just ask in general..just looking from physics perspective

We have a PhysicsSE that is a great place for physics questions.

Inertial and non-inertial reference frame? Interaction forces are independent of reference frame and inertial forces are only dependent on the reference frame.

Jan can you write your answer?

@AeronauticFreek forces you are looking for are not applicable to air particles, but to any dust particle. air particle moves in all directions, not only in the average movement and direction of air particles. See this question on physics.SE for more information.

@Jan Hudec,centripetal force at this particle cause radial pressure gradient,but what is with centrifugal force.can you explain inertial centrifugal force vs reactive centrifugal force  for this praticle,what is difference with these two forces and how they fit in each frame of reference?

@Jan Hudec, .How do you mean there is no such thing?.https://en.wikipedia.org/wiki/Reactive_centrifugal_force.. i also first time heard for these term 2 days before..

@AeronauticFreek, well, this terminology is confusing and often used slightly differently. Anyway, the centripetal force is the force on the astronaut from the shell. It is an interaction force, independent of reference frame. There is a reaction to it, acting on the shell, that you may call reactive centrifugal force. That is also interaction force, so also independent of reference frame. This is balanced by the ‘centripetal’ stress in the shell. In the non-rotating frame, only force on the astronaut is the centripetal one, so he rotates. …

… In the rotating frame, there is a centrifugal force on everything that fixes it so the astronaut is in place.

@ManuH. It is better to clarify that in this kind of mathematical analysis an "air particle" is treated as a sufficiently large volume that thermal molecular motion may be ignored, yet sufficiently compact as to appear arbitrarily small at the macroscopic scale.

@Aeronautic Freek if you are looking for balance of forces, you must have steady state.  This can be found in balance of the 4 forces of the entire aircraft.  Most simply put, the force of the air moving downwards (thus the reactive force of the plane upwards) = weight, the force of drag from this activity = thrust.

@GuyInchbald That's exactly why it should not be named "air particle" (e.g. a pressure can only be defined on a volume containing at least one particle).  I propose to name it "a small volume of air" or "a dust in the air streamline" or any other non-ambiguous term.

@ManuH Then you had better tell NASA. Their glossary at https://www.grc.nasa.gov/WWW/K-12/aerosim/LessonHS97/GLOSSARY.html defines a streamline as "A line depicting the flow path of a particle of air."

@GuyInchbald Done. I'm waiting for there answer.

In the inertial frame of the free airstream, the pressure force causes the particle to accelerate inwards. Centrifugal and centripetal forces do not play a direct role; there is no space station wall for centrifugal force to be exerted on, nor to exert a reactive centripetal force. The pressure force acts in the role of the centripetal force shown in the astronaut diagrams. The balancing reactive force is instead another pressure force, which acts on the airfoil where it is known as lift.
In the rotating frame of the air particle, inertial centrifugal force does come into play. The particle must by definition remain stationary within its reference frame, so all forces on it must now balance. The pressure force is thus balanced by the inertial centrifugal force. The airfoil lift no longer plays a direct role within this reference frame (having helped to create it).

Your first paragraph is perfect, the wing is pulled into its own low pressure area as well (also creating induced drag).  It concerns me that the OP even worries about "balanced centrifugal force".  On what, a tiny nanomite along for the ride on the air molecule? +2 for "where it is known as lift".

@RobertDiGiovanni As I noted in a comment to the main question, it is trivial to clarify that in this kind of mathematical analysis an "air particle" is treated as a sufficiently large volume that thermal molecular motion may be ignored, yet sufficiently compact as to appear arbitrarily small at the macroscopic scale.

can you write your answer?

Air flow striking the wing is deflected either up or down as the wing literally slices through the air.  This can be looked at as either the air hitting the wing (wind tunnel) or the wing hitting the air (actual flight).  Relatively, between air and wing, the effects are the same.
The bottom airflow is rather mundane, slowing and creating higher pressure under the wing, eventually leaving the higher pressure zone either downward, to the rear, or out the side (to form a vortex).
The top particle, pictured, has a more interesting time.  It is deflected upwards not by the wing, but the zone of higher pressure in front of the wing.  Logic would suggest that this pushes the wing down, so what happens next?  How do we get lift?
The trick is to shape the wing in such a way, and pitch it at such an angle, to have that particle drawn backwards and downwards for a greater distance than it was pushed up, resulting in net positive lift.
Within a certain Reynolds number range, positive pitch and camber create a zone of low pressure above the wing, which can be only maintained within a range of velocity and angle of attack.  If this is done correctly, our particle will be accelerated by pressure differential as it tries to fill the void.
A lifting wing at adequate airspeed will pass by the particle before it reaches the wing, repeating the same process with the next particle over and over again. When returning to its original position, relative to the wing chord, the original particle is where the wing was.
Its momentum will continue its movement below the wing with its fellow particles, creating "downwash".

I didnt mean on this..I mean to show free body diagram at this particle..centripetal force=radial pressure gradient which balance centrifugal force,,also this partcle accelrate so it has inertial force ...etc etc

If the particle is accelerating, forces are not in balance, though you may wish to capture the exact moment it stops moving upwards and begins downward movement.  This will vary, depending on leading edge design and wing camber.  Sort of trivial (they all will, to produce upper wing lift, finishing with more downward momentum), but why not have a go at drawing it, and show us what you have so far.

If you throw a ball up in the air, it is always accelerating downwards, though it's movement (velocity) goes to 0 for an instant before it falls.  The gravity vector force is unbalanced, drag vector forces would reverse.  Drag and gravity forces would eventually balance to reach steady state only if ball is allowed to reach "terminal velocity".


text:
How does traffic handoff between ZMA and Cuba work?
Cuba and the US have long had "frosty" relations.
With some limited easing of restrictions, there are now direct flights between Miami, FL (controlled by ZMA) and Havana, Cuba.
Does Cuba have an ARTCC?  Do they coordinate with ZMA for passing off flights?Has this been an ongoing system even before the recent easing of sanctions, or is it relatively new?

It's probably an ongoing system, since ZMA will have been handing off any international through traffic from Europe.

Direct flights from Miami to Havana aren't new. They have long been available for travelers authorized by the US government (family visits, organized tours for "educational" or "religious" reasons, etc...)

There are also many flights that overfly Cuba from the USA to other destinations in the South every day.  These are also not new and I have personally been doing them for years.  It just takes a "Cuban Overflight Permit" and then you are handed off to Cuba from Miami just like any other hand off.  How it is handled behind the scenes is a good question though.  So Yes, they have an ARTCC.  Yes they coordinate with ZMA for passing off flights.  Yes it is an existing system from before the easing of the sanctions and isn't new.  "How" does the handoff work?  I have no idea.

After some searching, I found a paper from Wikileaks from November 19th, 2009, that reads in part:

(SBU) SUMMARY:  Miami and Havana air traffic controllers
  signed a letter of agreement (LOA) on November 12 updating
  procedures for the coordination of flights between Cuban and
  U.S. controlled airspace.  Both sides stressed the excellent
  working relationship between the Miami Air Route Traffic
  Control Center (ARTCC) and the Havana Area Control Center
  (ACC).  The U.S. delegation agreed to review information to
  be provided by the Cuban delegation regarding flights
  operating under the rules of "due regard" that could pose a
  safety risk.  Both sides also agreed to discuss switching
  from verbal to electronic flight data sharing, on the margins
  of a regional aviation meeting in Miami in May 2010. END
  SUMMARY

Apparently handoff procedures have been a regular occurrence for a long time, and are cooperated on extensively.

As an early active air traffic controller in cuba I can tell that MIA ARTCC people were the best to work with,and jamaica the other end, meaning the worst ones,all guys in MIA had a great relationship with us ,of course havana had some bad apples regarding supervisor coordinations but other than everything was fine,they were our model to imitate  actualy met couple of controllers that went to Cuba and had a great time with them.

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